In trench type power semiconductor devices, such as trench power metal oxide semiconductor field effect transistors (MOSFETs) and insulated-gate bipolar transistors (IGBTs), on-state resistance (RDSon), breakdown voltage (BV) and output capacitance (Coss) are among some of the defining performance parameters. The on-state resistance is the drain-to-source resistance of the trench type power semiconductor device in an on-state, which depends partially on the depth of a source trench. The breakdown voltage is the voltage at which a reverse biased body-drift p-n junction breaks down and significant current starts to flow between a source and a drain of the trench type power semiconductor device by an avalanche multiplication process. The output capacitance can be dominated mated by a drain-to-source capacitance (CDS) and a gate-to-drain capacitance (CGD). The drain-to-source capacitance includes a capacitance associated with a source-to-drain overlap, which is an overlap of the conductive filler in the source trench and the semiconductor material between the source trench and the drain region. Having a high output capacitance can cause high ringing of a switch node and increase switching loss, thereby decreasing circuit efficiency.
It is desirable for a trench type power semiconductor device to have reduced RDSon and Coss, and still be able to withstand a high drain-to-source voltage during its off-state a high reverse voltage blocking capability). However, there are trade-offs among these parameters in the design of a trench type power semiconductor device. For example, in a conventional trench type power semiconductor device, while the on-state resistance and breakdown voltage are both related to the depth of the source trench, it is very difficult to improve both the on-state resistance and breakdown voltage by adjusting the depth of the source trench. In addition, in the conventional trench type power semiconductor device, impact ionization mainly occurs along the bottom and sidewalls of the source trench causing damage at the semiconductor-dielectric interface, which may lead to an increase in drain-to-source leakage and/or breakdown voltage degradation.
Accordingly, there is a need to overcome the drawbacks and deficiencies in the art by providing a power semiconductor device, such as a power MOSFET, with reduced on-state resistance and output capacitance without compromising the reverse voltage blocking capability (i.e. the breakdown voltage) of the power semiconductor device.